1. Field of the Invention
The present invention relates to an optical receiver used in an optical communication system to receive an optical signal.
2. Related Prior Arts
FIG. 6 shows an example of a conventional monitoring unit 104 installed within an optical receiver 100. The optical receiver 100 includes a converting unit 102, a processing unit 103, a pair of capacitors, 113a and 113b, to couple the converting unit 102 with the processing unit 103, and a resistor 131 for defining the input impedance of the processing unit 103. The converting unit 102 provides a photodiode (hereinafter denoted as PD) 121 for converting an optical signal PIN into a current signal IIN, a trans-impedance amplifier (hereinafter denoted as TIA) 122 for converting the current signal IIN into a voltage signal, and a buffer amplifier 123 configured to transform the output of the TIA 102 into a differential signal. The processing unit 103 includes a plurality of amplifiers, 132 and 133. The monitoring unit 104 takes the differential signal from the outputs of the first amplifier 132 in the processing unit 103.
The monitoring unit 104 includes a differential detector 104a, a reference generator 104b, and a comparator 104c. The differential detector 104a is configured to hold a peak level in a difference of the input signals and to generate an envelope MON corresponding to the variation of the peak level. Comparing this envelope MON with a reference REF generated in the reference generator 104b, the comparator 104c sets the output thereof when the output MON of the differential detector 104a is smaller than the reference REF, which is LOS_ALARM indicating that the optical signal PIN is lost, namely, Loss-Of-Signal alarm.
Various monitoring units have been well known. For example, the Japanese patent application published as JP-2003-152460A, has disclosed one type of the monitoring unit, in which the photocurrent generated by the PD is monitored through the current mirror circuit and a status of no optical signal is detected by comparing the voltage signal converted from thus detected photocurrent. Moreover, the optical receiver disclosed therein, the monitoring of the optical signal has been carried out from the current anterior to the high pass constituted by the capacitor coupling the converting unit and the processing unit and the resistor.
The other Japanese patent application published as JP-H07-095156A has disclosed another circuit to decide a loss-of-signal alarm. The circuit disclosed therein provides a level detector to generate an alarm to indicate the decrease of the optical input level and a consecutive identical detector to detect a state where the logical “0” level continues more than a predetermined number in the data recovered from the input signal.
The conventional monitoring unit 104 shown in FIG. 6 takes the signal from a position posterior to the coupling capacitor, 113a and 113b, and the resistor 131 to detect the peak level in the difference of the input signals. In such case, the coupling capacitor, 113a and 113b, and the resistor 131 constitute a high pass filter (hereinafter denoted as HPF), and cause a first order lag in the signal output from the converting unit 102. Additionally, when the optical receiver 100 has the TIA 122 with a unit to adjust the conversion gain thereof by varying the feedback resistance, a low-pass filter (hereinafter denoted as LPF) 124 to convert a single-ended output of the TIA 122 into a differential output, and a feed-forward circuit to cancel the input offset voltage of the processing unit 103, these particular circuits cause the second order lag to make the response of the monitoring unit 104 complex when the optical signal is shut down or recovered.
FIGS. 7A to 7D are time-charts of the optical receiving 100 when the optical signal PIN is shut down at tA. FIG. 7A shows an output of the converting unit 102, FIG. 7B shows an output from the HPF formed by the capacitors, 113a and 113b, and resistor 131, FIG. 7C shows an output MON of the differential detector 104a, and FIG. 7D is an output of the comparator 104c, namely, the LOS_ALARM.
The output of the converting unit 102, as mentioned previously, shows the first order lag due the LPF 124 to convert the single ended signal into the differential signal. Accordingly, the waveform of the output of the converting unit 102 after the time tA shows a gradual approach to the center level V0 from the lower and upper levels, as shown in FIG. 7A. The outputs, S+ and S−, from the HPF adds the second order lag, accordingly, the waveform after the time tA behaves as the second order response accompanying with overshoots and undershoots, as shown in FIG. 7B. The influence of the overshoot is left in the output MON from the differential detector 104a, as shown in FIG. 7C. When the overshoot in the output MON of the differential detector 104a exceeds the reference REF, from tB to tC in FIG. 7C, the alarm LOS_ALARM indicating the loss-of-signal may be temporarily reset despite the status with no optical signal continues.
The Japanese patent published as JP-2003-152460A detects the signal prior to the capacitor to avoid the first order lag due to the HPF. However, the circuit disclosed in the patent branches the output of the current-to-voltage converter, which degrades the high frequency performance of the optical receiver.
The present invention is to provide an optical receiver, in which the monitoring unit receives the signal posterior to the HPF, namely, the signal with the second order lag, that prevents the erroneous operation to temporarily reset the alarm indicating the no optical signal.